1. Field of the Invention
The present invention relates to a distributed Raman amplifier which supplies a pumping light to an optical transmission path to Raman amplify a wavelength division multiplexed (WDM) signal light, and a WDM optical transmission system using the distributed Raman amplifier, and in particular, to a technology for realizing an automatic gain control of the distributed Raman amplifier.
2. Description of the Related Art
In recent years, the setting up of photonic network by a WDM transmission technology has been practically realized. Further, there have been demanded the low-cost of total optical network by the extension of direct optical amplifying-repeating distance and the cost-reduction of network by the extension of regenerative repeating distance, and accordingly, the development of ultra-long distance of optical amplifying-repeating transmission has been progressed. On the other hand, with the speeding-up of transmission speed per wavelength (channel) from the existent 10 Gb/s to 40 Gb/s, there is also a trend to aim at the further low noise of optical amplification repeater, and for this purpose, further low noise optical amplification repeating devices need to be used. In a trunk WDM optical transmission system, a distributed Raman amplification technology is used in order to significantly improve the transmission path performance, and a distributed Raman amplifier (DRA) is one of low noise optical amplification repeating devices.
FIG. 7 is a diagram showing one example of a conventional WDM optical transmission system using the DRA. In this conventional WDM optical transmission system, a WDM signal light generated by an optical transmission apparatus (Tx) 101 is transmitted to an optical transmission path 102, to be transmitted up to an optical reception apparatus (Rx) 104 via a plurality of optical repeaters 103 arranged on the optical transmission path 102. Further, to the optical transmission path 102 in each repeating section, a pumping light Lp for Raman amplification which is output from a pumping light source (LD) 105 is supplied via a multiplexer 106, so that the WDM signal light propagated through the optical transmission path 102 in each repeating section is subjected to the distributed Raman amplification to be input to the latter stage optical repeater 103.
FIG. 8 is a diagram showing a configuration example of the DRA applied to each repeating section of the above conventional WDM optical transmission system. In the DRA 110 shown in this configuration example, pumping lights Lp1, Lp2 and Lp3 of different wavelengths are output from three pumping light sources 111, 112 and 113, and the respective pumping lights Lp1 to Lp3 are multiplexed by a multiplexer 114 to one pumping light to be supplied onto the optical transmission path 102 via a multiplexer 115. Further, on the latter stage of the multiplexer 115, a fixed gain equalizer (GEQ) is disposed for compensating for gain wavelength characteristics of the distributed Raman amplification in the optical transmission path 102.
It is known that, when an average gain of the distributed Raman amplification is changed, the gain wavelength characteristics thereof are also changed. FIG. 9 and FIG. 10 are diagrams each showing one example of gain wavelength characteristics change in the conventional DRA 110 shown in FIG. 8. To be specific, the gain wavelength characteristics of the DRA 110 are such that single-peaked gain bands, which respectively have peaks at wavelengths obtained by Raman shifting the respective wavelengths of the three pumping lights Lp1 to Lp3, are overlapped with each other, as shown in FIG. 9, and in shapes of the single-peaked gain bands, as the average gain becomes larger, changes in convexo-concave shapes corresponding to the pumping lights Lp1 to Lp3 become remarkable, as shown in FIG. 10.
Therefore, in the case where the gain wavelength characteristics of the DRA are compensated using the fixed GEQ as in the configuration example of FIG. 8, since only any one of the gain wavelength characteristics shown in FIG. 10 can be completely compensated, wavelength characteristics of the WDM signal light output from the GEQ are changed depending on an operating state of the DRA, and consequently, transmission characteristics of the distributed Raman amplified WDM signal light are degraded. In order to avoid such degradation, it is required to control the average gain of the DRA at constant. As means for simply realizing an automatic gain control of the DRA, it is considered that the supply power of the pumping light is controlled at constant.
However, according to the above realizing means, even when fiber characteristics, such as loss wavelength characteristics and the like, are changed due to the deterioration of optical fiber used for the optical transmission path, the DRA is operated at the constant pumping light power, and therefore, the gain wavelength characteristics of the DRA are changed, resulting in an error in the compensation by the fixed GEQ. Further, since Raman amplification efficiency is changed depending on types of optical fibers used for the optical transmission path, if the gain is changed for each fiber type of the optical transmission path, the gain wavelength characteristics of the DRA are changed, resulting in the error in the compensation by the feed GEQ, similarly to the above, which causes the degradation of transmission characteristics.
As a conventional technology for the automatic gain control of the DRA, in Japanese Unexamined Patent Publication No. 2004-193640, there has been proposed a technology for sending a reference light, which is not Raman amplified, to the optical transmission path together with a signal light of plural wavelengths; distributed Raman amplifying the signal light on the optical transmission path, demultiplexing the reference light from the light propagated through the optical transmission path to detect a level thereof; and calculating a control target value of a signal light level based on the level of the reference light to control the pumping light, thereby realizing a constant average gain and constant wavelength characteristics, irrespectively of the optical fiber or the signal light level.
However, in the conventional technology for controlling the pumping light utilizing the reference light which is not Raman amplified, since a wavelength of the reference light is set in a region considerably separated from a wavelength band of the signal light, characteristics, for example, a loss coefficient of the optical transmission path and the like, are different between the signal light and the reference light, so that a significant error occurs between the signal light level and the reference light level at an output point of the optical transmission path. Therefore, there are problems in that, even if the pumping light is controlled based on the reference light level detected at the output point, it is difficult to realize with high precision the automatic gain control of the DRA, and, accordingly, it is hard to compensate for the gain wavelength characteristics of the DRA by the fixed GEQ irrespectively of types, characteristic changes or the like of fibers used for the optical transmission path.